This invention relates to the problem of combining the outputs from a group of single mode optical waveguides, typically optical fibres, efficiently upon the photosensitive surface of a photodetector.
Such a requirement may be found for instance in certain types of access network where a single photodetector is required to detect the output of any selected one from a set of n single mode fibres. Such a situation may arise for instance in fibre-in-the-home applications. Another application, one which will typically require a much faster photodetector, is in the construction of an ATM switch of high capacity from the optical interconnection of a set of smaller switches. Such a switch is for instance the subject of our co-pending application GB 2 289 813A in which the combining function is provided by a 1 to 16 integrated single mode waveguide radiative star arrangement which is operated in reverse direction so as to act as a combiner rather than as a splitter. It could alternatively have been provided by a tree of bifurcating splitters, similarly operated in reverse. In either instance the combining function is achieved with no insignificant loss: in the case of the tree structure this would be not less than 12 dB for a 1 to 16 tree.
An approach to providing reduced loss is described in U.S. Pat. No. 5,136,677. As described with particular reference to its FIG. 2, the approach involves progressively etching away most of the cladding glass from near one end of each of a set of fibres. These fibres are assembled into a bundle which is inserted through a silica tube that is dimensioned to freely receive the bundle. A quantity of potting compound is introduced into the bore of the tube. In this way the fibre ends are brought into a somewhat more tightly packed bundle that permits the free engagement around it of a second tube. This second tube has a smaller bore than that of the first and is dimensioned to fit inside the bore of the first tube. The function of the second tube is to bring the fibre ends into a still more tightly packed bundle than is achieved by the first tube alone. If necessary, one or more further tubes can be fitted, each nesting inside the end of its predecessor, each engaged freely around the bundle and protruding from that predecessor, and each serving to bring the fibres into an ever more tightly packed state than is achieved by the use of the preceding tube.
The present invention is directed to an assembly incorporating a combiner which exhibits significantly less loss than the equivalent tree or radiative star combiners described above, and achieves this objective without having to have recourse to etching of cladding glass from the individual waveguides, thereby avoiding the particular fragility problems associated without such etching and subsequent assembly of the etched fibres.
According to the present invention there is provided an assembly including a close-packed bundle of optical fibres having adiabatically tapered fibre cores, which fibres are single mode at the large end of the taper, wherein the small end of the taper is optically coupled with one end of a length of multimode fibre the other end of which multimode fibre is optically coupled with the photosensitive area of a photodetector.
The tapering process of the present invention is relatively easy to implement in a controlled way using progressive stretching. The fibres do not have to be separately tapered, but can all be tapered together once the fibres have been assembled into a bundle. In this way all the fibres of the bundle are subjected to identical tapering without engendering problems of excessive fragility. Furthermore the tapering can be employed to expand the modal spot size of, and hence reduce the far-field divergence of, the light issuing from the small end of the taper. This reduced divergence facilitates imaging on a small area, and hence fast, detector without excessive loss occasioned by numerical aperture limitations.